1. Field of the Invention
This invention relates to a Cu—Ni—Si-based copper alloy sheet material suitable for use in electrical and electronic parts such as connectors, lead frames, relays, switches and the like, particularly to a copper alloy sheet material that exhibits excellent bending workability and stress relaxation resistance property while maintaining high strength and high conductivity, and a method of manufacturing the same.
2. Background Art
The connectors, lead frames, relays, switches and other current-carrying components of electrical and electronic parts require good conductivity for minimizing generation of joule heat due to passage of current and also require high strength capable of enduring stress imparted during assembly and/or operation of the electrical or electronic parts. Because electrical and electronic parts are generally formed by bending, the current-carrying components must also have excellent bending workability. Moreover, in order to ensure contact reliability between the electrical and electronic parts, they require endurance against the tendency for contact pressure to decline over time (stress relaxation), namely, they need to be excellent in stress relaxation resistance property.
Of particular note is that as electrical and electronic parts have become more densely integrated, smaller, and lighter in weight in recent years, demand has increased for thinner copper and copper alloy materials for use in the parts. This in turn has led to still severer requirements regarding the level of material strength. To be more specific, a strength level expressed as tensile strength of 700 MPa or greater, preferably 750 MPa or greater, is desired.
Further, the emergence of smaller and more complexly shaped electrical and electronic parts has created a strong need for improved shape and dimensional accuracy in components fabricated by bending. Recently, therefore, increased use is being made of a bending method in which the starting material is notched at the location to be bent and bending is later carried out along the notch (sometimes called the “notch-and-bend method” in the following). With this method, however, the notching work-hardens the vicinity of the notch, so that cracking is apt to occur during the ensuing bending. The notch-and-bend method can therefore be viewed as a very harsh bending method from the viewpoint of the material.
In addition, the fact that more and more electrical and electronic parts are being utilized in severe environment applications has made stress relaxation resistance property an increasingly critical issue. Stress relaxation resistance property is of particular importance when the part is exposed to a high-temperature environment as in the case of an automobile connector. “Stress relaxation” refers to the phenomenon of, for instance, a spring member constituting an element of an electrical or electronic part experiencing a decline in contact pressure with passage of time in a relatively high-temperature environment of, say, 100 to 200° C., even though it might maintain a constant contact pressure at normal temperatures. It is thus one kind of creep phenomenon. To put it in another way, it is the phenomenon of stress imparted to a metal material being relaxed by plastic deformation owing to dislocation movement caused by self-diffusion of atoms constituting the matrix and/or diffusion of solute atoms.
But there are tradeoffs between strength and conductivity, strength and bending workability, and bending workability and stress relaxation resistance property. Up to now, the practice regarding such current-carrying components has been to take the purpose of use into account in suitably selecting a material with optimum conductivity, strength, bending workability or stress relaxation resistance property.
Cu—Ni—Si-based alloy (known as Corson alloy) has attracted attention in recent years for its excellent balance between strength and conductivity. Copper alloy of this type can be markedly improved in strength while still retaining relatively high conductivity (of 30% to 45% IACS). However, Cu—Ni—Si-based alloy is known to be an alloy system that is difficult to make excellent in both strength and bending workability or both bending workability and stress relaxation resistance property.
Strength can be increased by such commonplace methods as adding a greater amount of solute elements like Ni and Si and increasing the rolling reduction ratio following aging treatment. However, the former method reduces conductivity and causes bending workability to decline with increasing amount of Ni—Si type precipitates. The latter method increases work-hardening, thereby degrading bending workability (particularly bending workability perpendicular to the rolling direction, i.e., bending workability with respect to a bending axis lying parallel to the rolling direction). Thus while a high strength level and a high conductivity level may be achieved, it may become impossible to form the electrical or electronic part.
A method commonly used to avoid a decrease in bending workability is to omit (or minimize) post-aging finish cold rolling and make up for the strength loss this causes by adding large amounts of solute elements such as Ni and Si. However, this method increases the tendency of work-hardening for the material, so that when the notch-and-bend method is adopted, the notching markedly increases hardness in the vicinity of the notch. A problem therefore arises of the bending workability being radically degraded at the time of bending the material along the notch.
Refinement of crystal grain size effectively improves bending workability. So it is a common practice to carry out the solution heat treatment of the Cu—Ni—Si-based alloy not in a high-temperature region so that all precipitates (or crystallization products) enter into solid solution but in a relatively low-temperature region so that some precipitates (or crystallization products) remain to have a pinning effect on recrystallization grain growth. However, while it may be possible to achieve crystal grain refinement in this case, the amount of Ni and Si entering solid solution is reduced, which inevitably lowers the strength level after aging treatment.
Moreover, the crystal grain boundary area per unit volume increases with decreasing crystal grain diameter. Crystal grain refinement therefore promotes stress relaxation, which is a type of creep phenomenon. Particularly in the case of vehicle-mounted connectors and other high-temperature environment applications, the diffusion velocity of the atom along grain boundaries is extremely high than that within the grains, so that the loss of stress relaxation resistance property caused by crystal grain refinement becomes a major problem.
In recent years, control of crystal orientation (texture) has been proposed for improving the bending workability of Cu—Ni—Si-based alloys (see patent documents 1 to 5).                Patent Document 1: JP2000-80428A        Patent Document 2: JP2006-9108A        Patent Document 3: JP2006-16629A        Patent Document 4: JP2006-9137A        Patent Document 5: JP2006-152392A        
It is well known that crystal grain refinement and control of crystal orientation (texture) are effective for improving the bending workability of copper alloy sheet material. Regarding control of the crystal orientation (texture) of Cu—Ni—Si-based copper alloy, in the case where ordinary manufacturing processes are utilized, the X-ray diffraction pattern from the sheet surface (rolled surface) is generally dominated by diffraction peaks from the four crystal planes {111}, {200}, {220} and {311}, and the X-ray diffraction intensities from other crystal planes are very weak compared with those from these four planes. The diffraction intensities from the {200} plane and the {311} plane are usually large after solution heat treatment (recrystallization). The ensuing cold rolling lowers the diffraction intensities from these planes, and the X-ray diffraction intensity from the {220} plane increases relatively. The X-ray diffraction intensity from the {111} plane is usually not much changed by the cold rolling.
In order to improve bending workability, Patent Document 1 defines the ratio of the sum of the X-ray diffraction intensities from the {200} plane and the {311} plane to the X-ray diffraction intensity from the {220} plane as:(I{200}+I{311})/I{220}>0.5.
This relational expression suggests that bending workability improves when the reduction ratio in the cold rolling conducted after solution heat treatment is lowered. This kind of texture regulation usually lowers strength. And, in fact, tensile strength of the copper alloy provided by Patent Document 1 is only on the order of 560 to 670 MPa.
Patent Documents 2 and 3 point out that the fact that bending workability is anisotropic makes it difficult to improve bending workability simultaneously both for the case where the bending axis lies perpendicular to the rolling direction (G.W.) and for the case where it lies parallel to the rolling direction (B.W.). It therefore separately defines means for improving G.W. bending workability and means for improving B.W. bending workability. That is, the former means is to make the ratio of the sum of the X-ray diffraction intensities from the {111} plane and the {311} plane to the X-ray diffraction intensity from the {220} plane, i.e., (I{111}+I{311})/I{220}, not greater than 2.0 and the latter means is to make the ratio not less than 2.0.
In order to improve bending workability, Patent Document 4 defines the X-ray diffraction intensities from the {311} plane, {220} plane and {200} plane as a function of crystal grain diameter A, as follows:I{311}×A/(I{311}+I{220}+I{200})<1.5.Patent Document 5 defines the percentage of cube orientation [{001}<100>] as 50% or greater and the average crystal grain diameter as 10 μm or less. These techniques require crystal grain refinement. The stress relaxation resistance property generally decreases in such cases.
Use of the aforesaid notch-and-bend method on a copper alloy sheet material effectively improves the shape and dimensional accuracy of the bent product. However, even in the Cu—Ni—Si-based alloys improved in bending workability by texture control as in Patent Documents 1 to 5, no consideration is given to preventing cracking caused by the notch-and-bend method, indicating that the post-notching bending workability is not sufficiently improved.
Moreover, while, as mentioned in the foregoing, crystal grain refinement is effective for improving bending workability, it is a negative factor with regard to overcoming stress relaxation, which is one type of creep phenomenon. Because of this, and the fact that achieving a high degree of improvement is difficult even with to regard bending workability alone, still further improvement of stress relaxation resistance property cannot be achieved even by using the prior art texture control.